mtd: lantiq-flash don't specify default parsing options

Since 'cmdline, NULL' is now a default for parse_mtd_partitions, don't specify
this in every driver, instead pass NULL to force parse_mtd_partitions
to use default.

Artem: tewaked the patch

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>